Handover with mobile relays

ABSTRACT

A method to provide mobility support of UEs ( 12 ) that are served by mobile relay base stations ( 14 ), by providing handover of a UE ( 12 ) for data traffic only between two mobile relay base stations. This is achieved by information exchanging between the source and target mobile relay stations of the concerned UE in handover, and/or the corresponding donor base stations ( 13, 11 ) if applicable. The handover is triggered by any of the following: based on UE&#39;s measurement report, the signal quality of a neighbour cell is better than the current serving cell; based on the measurement of the serving mobile relay base station, the signal quality of the link to the UE is worse than a certain threshold; or based on the measurement of the serving mobile relay base station, the signal quality of a neighbour cell (potential donor base station) is much better than the current donor base station.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent ApplicationNo. PCT/EP2013/066006, filed Jul. 30, 2013, now pending, and claimspriority to European Patent Application No. 12183655.5, filed Sep. 10,2012, the contents of each are herein wholly incorporated by reference.

FIELD OF THE INVENTION

This invention generally relates to wireless communication systems andin particular, to a handover method in such systems.

BACKGROUND OF THE INVENTION

Wireless communication systems are widely known in which subscriberstations communicate wirelessly in cells provided by base stations. Nextgeneration wireless communications systems such as the Universal MobileTelecommunications System (UMTS) and the UMTS Long Term Evolution (LTE)aim to offer improved services to the user compared to the existingsystems. These systems are expected to offer high data rate services forthe processing and transmission of a wide range of information, such asvoice, video and IP multimedia data.

The subsequent description will refer to LTE by way of example, and thebasic system architecture in LTE is illustrated in FIG. 1. In LTE thesubscriber stations are referred to as user equipments (UEs) and thebase stations are called enhanced NodeBs (eNBs). As can be seen, each UE12 connects over a wireless link via a Uu interface to an eNB 11, whichdefines one or a number of cells for wireless communication. There is anetwork of eNBs referred to as the “eUTRAN”.

Each eNB 11 in turn is connected by a (usually) wired link using aninterface called S1 to higher-level or “core network” entities,including a Serving Gateway (S-GW 22), and a Mobility Management Entity(MME 21) for managing the system and sending control signalling to othernodes, particularly eNBs, in the network. In addition, a not-shown PDN[Packet Data Network] Gateway (P-GW) is present, separately or combinedwith the S-GW 22, to exchange data packets with any packet data networkincluding the Internet. The wired links to the core network EPC (whereEPC stands for Evolved Packet Core) are referred to as “backhaul” andemploy Internet Protocol (IP).

As is implied by the above distinction between MME/S-GW and PDN-GW, inLTE, control signalling is separated from user data traffic; thus thereis a “user plane” which is distinct from the “control plane”. FIG. 1,the S1 interface is labelled S1-U, the suffix—U denoting the user planeemployed by the eNBs 11 for communicating user data to and from the S-GW22. The S-GW is responsible for packet forwarding of user data on thedownlink to the UE 12 and on the uplink. The S-GW 22 provides a“mobility anchor” for the user plane during handovers of a UE 12 fromone eNB 11 to another.

In parallel to this, there is an interface S1-MME (sometimes calledS1-C) via which the eNBs 11 exchange control messages with the MME 21.The main function of the MME 21, as its name suggests, is to managemobility of the UEs 12, and it is a signalling-only entity; in otherwords, user data packets do not pass through the MME. The MME 21 is alsoresponsible for controlling security (including authenticating users),and for EPS bearer control (see below). In practice, there may beseveral MMEs forming a MME “pool”. One eNB can have several S1-MMEinterfaces towards several MMEs.

In addition, as shown in FIG. 1, the eNBs 11 communicate amongthemselves by a (usually) wireless link, using an interface called X2for mutual co-ordination, for example when handing over a UE 12 from onecell to another. There is only one X2 interface between two eNBs.

In the above configuration, communications among eNBs can be regarded ascommunications among peers (network nodes at the same hierarchicallevel) with the MME constituting a higher level entity in the system.

To support high data rates in next-generation wireless communicationsystems, relay nodes may be employed as capacity boosters between thesubscriber stations and the base stations, or in other words (in thecase of LTE) between UEs and eNBs. So-called mobile relay base stationsare one possible type of relay node. Mobile relay base stations providethe same functionality as conventional base stations, but their link tothe network is provided by using a similar radio interface as that usedby UEs. In other words a mobile relay base station connects to an eNB(called a “donor” eNB or DeNB in LTE) in a similar way as an ordinaryUE.

As the name suggests, a mobile relay base station is expected to havefull base station functionality, in particular the ability to handleboth user plane and control plane traffic. As the name also suggests, amobile relay base station is mobile, in other words it can be assumed tomove with a certain speed relative to the eNBs (which are generallyfixed) and possibly also with respect to at least some UEs to which itis connected. Consequently, as the mobile relay base station moves ithas to be handed over from a serving DeNB to another DeNB in order tomaintain a continuous connection to the network. Since the mobile relaybase station is expected to handle both control plane and user planetraffic, handover of a mobile relay base station conventionally involveshandover of both types of traffic.

The basic LTE system architecture with a mobile relay base station isshown in FIG. 2. A UE 12 is connected to a mobile relay base station(labelled Mobile Relay 14 in FIG. 2) by a wireless link using the Uuradio interface. The Mobile Relay 14, in turn, connects to a DeNB 13over a wireless link via the Un interface. The Donor (also calledAnchor) eNB 13 may serve one or more mobile relays 14 (as well aspossibly relay nodes of other kinds) and may also communicate directlywith other UEs.

The User Plane data for UE 12 (user data traffic) is routed to the S-GW(labelled Serving Gateway 22). Typically the S-GW is used for severaleNBs which may be interconnected by the X2 interface, which may be areal physical connection between the eNBs, or implemented as a logicalconnection via other network nodes. The DeNB 13 is the eNB that isconnected to the Mobile Relay 14 using the radio interface (Un) andwhich uses similar radio resources to the Uu radio interface.

Although the Mobile Relay is treated like an eNB to some extent, andthus needs to send and receive S1-AP and/or X2-AP signalling, as isclear from FIG. 2 the S1 (and possibly X2) interface is carried betweenthe RN and its DeNB over the Un interface.

Transmission of messages between nodes in a radio network, such asbetween mobile relays and eNBs, involves the use of multi-layer protocolstacks. On the transmission side, starting from the top of the stack atan application layer, each layer in the protocol stack processes unitsof data (packets) in some way, usually adding a header to the data unitbefore passing it down to the next lower layer or sub-layer. The headersinclude fields identifying the operations performed at that protocollayer. On the reception side, each layer decodes the header inserted inthe corresponding transmission-side layer to allow reconstruction of adata unit, which is then passed up to the next higher layer.

FIG. 3 shows the protocol stacks in LTE for (1) User Plane and (2)Control Plane.

In the User Plane, user data traffic is transported via the two radiointerfaces (Uu and Un). The User-Plane consists of Packet DataConvergence Protocol (PDCP), Radio Link Control (RLC), Medium AccessControl (MAC) and PHYsical (PHY) protocol layers. At the PDCP protocollayer, one protocol of particular relevance for present purposes isGTP-U, which is used on the S1 interface between the eNB and S-GW and onthe S5/S8 interface between S-GW and P-GW. GTP stands for GPRS TunnelingProtocol and allows user data packets to be conveyed (“tunnelled”)between the P-GW and eNB.

The concept of “bearers” is important for achieving quality-of-service(QoS) in a packet-based network such as LTE. In general, a “bearer” canbe thought of as an information transmission path of defined capacity,delay and bit error rate, etc. so as to enable a given service orcontrol function to be provided. Various types or levels of bearer canbe established, the radio part being set up using radio resource controlor RRC. A single UE may have associated with it multiple bearerssimultaneously for providing different services to the user.

FIG. 4 shows an EPS Bearer Service Architecture employed in LTE. Theleft side of the Figure represents the eUTRAN with the EPC occupying themiddle part of the Figure. At the right-hand side, outside the LTEsystem as such, there is the Internet. The vertical bars represent themain entities in the user plane, from the UE 12 to eNB 11 through toS-GW 22 and P-GW 23, terminating in a peer entity 24 (such as anInternet web server) connected to the P-GW 23. Communication takes placebetween the S-GW and P-GW over an S5 or S8 interface. To provide anend-to-end service between the UE 12 and Peer Entity 24 (as indicated bythe upper horizontal band in the figure), the system sets up “bearers”as shown. An EPS Bearer represents the entire connection within the LTEsystem; it constitutes a QoS flow for a particular service. Theconnection continues outside the LTE system via an External Bearer.

The EPS Bearer is made up, in turn, of a radio bearer over the linkbetween the UE 12 and eNB 11, and an S1 Bearer between the eNB 11 andS-GW 22. A further Bearer (S5/S8 Bearer) is set up between the S-GW 22and P-GW 23. Each Bearer can be regarded as a “tunnel” in a givenprotocol layer for transport of packets, connecting the end points forthe duration of a particular service or “session”, e.g. voice call ordownload. Thus, the radio bearer transports the packets of thehigher-layer EPS Bearer between the UE 12 and eNB 11, and the S1 Bearertransports the packets of the EPS Bearer between the eNB 11 and S-GW 22.Bearer control through RRC, mentioned previously, includes the settingup of bearers for a particular session so as to ensure sufficient QoS,taking into account the resource situation in the E-UTRAN and existingsessions already in progress. It also involves the modification andrelease of radio bearers.

Further details of handover and signalling procedures in LTE arecontained in the following documents which are hereby incorporated byreference:—

3GPP TS36.300 “Evolved Universal Terrestrial Radio Access (E-UTRA) andEvolved Universal Terrestrial Radio Access Network (E-UTRAN); Overalldescription; Stage 2”

3GPP TS36.331 “Evolved Universal Terrestrial Radio Access (E-UTRA);Radio Resource Control (RRC); Protocol specification”

3GPP TS36.413 “Evolved Universal Terrestrial Radio Access (E-UTRA) andEvolved Universal Terrestrial Radio Access Network (E-UTRAN); S1Application Protocol (S1-AP)”

3GPP TS36.423 “Evolved Universal Terrestrial Radio Access (E-UTRA) andEvolved Universal Terrestrial Radio Access Network (E-UTRAN); X2Application Protocol (X2-AP)”

As already mentioned, in a wireless communication system employingmobile relays as in FIG. 2, there is a requirement for handover of themobile relays and correspondingly of the UEs connected to those mobilerelays. Mobility management functions in LTE and UMTS networks for UEsin connected mode handle all necessary steps for handover. These stepsinclude processes that precede the final HO handover decision on thesource network side (control and evaluation of UE and base stationmeasurements), preparation of resources on the target network side,commanding the UE to the new radio resources and finally releasingresources on the source network side. Handover of UEs involvestransferring, from one eNB to another, all the information related tothe UE, called its “context”. Mobility management also containsmechanisms to transfer context data between the eNBs, and to update noderelations on both control plane and user plane.

A typical handover procedure in LTE networks, taken from the abovementioned 3GPP TS36.300, is illustrated in FIG. 5, showing three phasesof handover respectively labelled “Handover Preparation”, “HandoverExecution” and “Handover Completion”.

Suppose that a connected-mode UE 12 (or likewise a mobile relay basestation acting as a UE towards its donor base station) is connected to aSource eNB 11 providing a serving cell, and can receive at leastreference signals from a neighbour cell provided by a Target eNB 11. Ina step 1. “Measurement Control”, the UE 12 is triggered to sendmeasurement report by the rules set by i.e. system information,specification etc. (see 3GPP TS36.331). In a step 2. “MeasurementReports”, UE 12 performs measurements of attributes of the serving andneighbour cells. Step 3 “HO decision” is for Source eNB 11 to make adecision based on measurement report and RRM information to hand overthe UE 12. Then, (4. “Handover Request”) the Source eNB issues ahandover request to the Target eNB, passing necessary information toprepare the handover at the target side. In a step 5. “AdmissionControl”, admission control may be performed by the Target eNB todetermine whether or not it agrees to accept the UE. Then (6. “HandoverRequest Ack.”) the Target eNB 11 prepares the HO and sends the handoverrequest Ack. to the Source eNB, in which a handover command is includedfor the Source eNB to forward the command in the form of a messagelabelled “7. RRC Conn. Reconf.mobilityControlinfo”, to instruct the UEto connect to the target cell.

Several necessary steps are performed on the network side to ensure alossless user plane path switch, in other words minimum interruption inthe data packets being transmitted to or from the UE. These include astep 8. “SN Status Transfer” by which the Source eNB informs the TargeteNB of the Sequence Number (SN) up to which it has successfullydelivered data packets, in order for the Target eNB to know at whichpacket to start transmission.

After receiving the handover command, UE performs synchronisation toTarget eNB (9. “Synchronization”) and accesses the target cell. TheTarget eNB responds (10. “UL Allocation+TA for UE”) with uplinkallocation and timing advance. When the UE has successfully accessed thetarget cell, the UE sends a message (11. “RRC Connection ReconfigurationComplete”) to the Target eNB 11 to confirm the completion of handover.

The subsequent steps 12.-15. in FIG. 5 can be summarised as a user planepath switch, which changes the DL user plane data delivery path from thepath: S-GW->Source eNB to: S-GW->

Target eNB. Finally, the MME 21 confirms (16. Path Switch Req. Ack) thehandover and the Target eNB 11 then sends a message (17. “UE ContextRelease”) to instruct the Source eNB to release the resources previouslyallocated to the handed-over UE.

As will be understood, the above handover is a handover of both thesignalling and data traffic, or in other words both the control planeand user plane. This is what is normally understood by “handover” in awireless communication system.

The problem addressed by this invention is that in systems such as LTEand UMTS, mobile relay base stations are deployed in certain areas, forexample, peak hours in city centres. Mobile relays installed in vehicles(such as, cars, buses) are provided primarily for the use of UEs ofpassengers of those vehicles. However, such mobile relays move withrelatively low speed, which also allows other users such as pedestrians(outside of the vehicles) with low speed to access these mobile relays.Thus, in this scenario, mobile relays are deployed as capacity boostersserving UEs which have a low relative speed relative to the mobilerelays. The key issue in this scenario is how to support handovers ofthe UEs which are necessitated by the movement either of mobile relaysor of UEs.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda handover method in a wireless communication system having a subscriberstation, at least one base station and a plurality of relay stations,the subscriber station exchanging data traffic and signalling byemploying wireless links with the base station, said wireless linksincluding relay links which are indirect links between the subscriberstation and the base station via one or more of the relay stations, themethod comprising:

-   -   measuring a signal quality of each of a plurality of said        wireless links;    -   determining a need for a handover of the subscriber station from        a serving relay station to another relay station by comparing        the signal quality of a said relay link with a predetermined        threshold and/or with the signal quality of another said        wireless link; and    -   performing a handover of the subscriber station from said        serving relay station to said another relay station;        characterised by:    -   employing, for control signalling between the subscriber station        and the base station, a said wireless link which is a direct        link between the subscriber station and the base station; and    -   performing said handover for data traffic only whilst        maintaining said direct link for control signalling.

The “measuring” referred to above may be performed either at thesubscriber station or at the serving relay station, or both, asexplained below.

The relay stations referred to above are of a type having base stationfunctionality at least to the extent of allowing separation of userplane and control plane functions. Each relay station preferably definesits own cell(s) and thus appears like a base station to the subscriberstation. Preferably, at least the serving relay station is mobile andthe base station acts as a donor base station for the relay stations.Thus, in LTE terminology, the relay station may be referred to as a“mobile relay base station”, and the base station as a donor eNB.

Various embodiments of the method are provided, which differ in thetrigger for the handover.

In the first embodiment the measuring is performed at least at thesubscriber station and the determining is performed at the base stationin response to reports on said signal quality received from thesubscriber station.

In second and third embodiments the measuring is performed at least atthe serving relay station and the determining is performed at the basestation in response to reports on said signal quality received from theserving relay station. Of course, in the second and third embodimentsthe subscriber station may also measure the signal quality of itswireless links; however these measurements are not decisive for thehandover decision.

In the first embodiment, preferably, the determining determines whetherthe signal quality of a wireless link of the subscriber station withanother relay station exceeds by a predetermined threshold the measuredsignal quality of a wireless link between the subscriber station and theserving relay station.

In the second embodiment, preferably, the determining determines whetherthe measured signal quality of a wireless link of the serving relaystation with the subscriber station is worse than a predeterminedthreshold.

In the third embodiment, preferably, the determining determines whetherthe signal quality of a wireless link of the serving relay station withanother base station exceeds by a predetermined threshold the signalquality of a wireless link between the serving relay station and thedonor base station.

The third embodiment may further comprise the relay station which wasthe serving relay station itself performing a handover from said basestation to said another base station.

In any method as defined above, performing a handover may include thebase station identifying said another relay station as a target relaystation, sending a cell activation request to the target relay stationincluding a context for the subscriber station and identifying the relaystation which currently acts as a serving relay station for data trafficof the subscriber station.

The further procedure may include:

-   -   the target relay station performing admission control of the        subscriber station;    -   the base station sending a deactivation request to the serving        relay station;    -   the base station sending a data traffic only handover command to        the subscriber station and a request to the subscriber station        to connect to the target relay station for data traffic        transmission;    -   the serving relay station sending a status report to the target        relay station;    -   the serving relay station forwarding data traffic to the target        relay station; and    -   the subscriber station detaching from the serving relay station        and connecting to the target relay station for data traffic        transmission.

According to a second aspect of the present invention, there is provideda wireless communication system having a subscriber station, at leastone base station and a plurality of relay stations, the subscriberstation arranged for exchanging data traffic and signalling by employingwireless links with the base station, said wireless links includingrelay links which are indirect links between the subscriber station andthe base station via one or more of the relay stations, the systemcomprising: measurement means for measuring a signal quality of each ofa plurality of said wireless links; handover determination means fordetermining a need for a handover of the subscriber station from onerelay station to another relay station by comparing the signal qualityof a said wireless link with a predetermined threshold and/or with thesignal quality of another said wireless link; and handover performingmeans for performing a handover of the subscriber station from said onerelay station to said another relay station; characterised in that: thesubscriber station is arranged to employ a direct link between thesubscriber station and the base station for control signalling betweenthe subscriber station and the base station; and said handoverperforming means is arranged for performing a handover for data trafficonly whilst maintaining said direct link for control signalling.

Thus, embodiments of the present invention involve the concept of a“data plane handover” which is different from handovers conventionallyemployed in a wireless communication system. In one form of data planehandover, the user plane data delivery path between the S-GW and donoreNB remains the same while the user plane data delivery path changesfrom the donor eNB to a new mobile relay base station.

According to a third aspect of the present invention, there is provideda subscriber station for use in the system as defined above.

According to a fourth aspect of the present invention, there is provideda relay station for use in the system.

According to a fifth aspect of the present invention, there is provideda base station for use in the above wireless communication system.

According to a further aspect of the present invention, there isprovided software which, when executed by a processor of a base station,a mobile relay station and/or a subscriber station in a wirelesscommunication system, performs any method as defined above. Suchsoftware may be stored on a computer-readable medium.

To summarise, embodiments of the present invention may provide awireless communication method to provide mobility support of UEs thatare served by mobile relay base stations, by providing handover of userequipment for data traffic only between two base stations. This isachieved by information exchanging between the source and target mobilerelay stations of the concerned UE in handover, and/or the correspondingdonor base stations if applicable. The handover is triggered by any ofthe following situations: (i) based on UE's measurement report, thesignal quality of a neighbour cell is much better than the currentserving cell; (ii) based on the measurement of the serving mobile relaybase station, the signal quality of the link to the UE is worse than acertain threshold; or (iii) based on the measurement of the servingmobile relay base station, the signal quality of a neighbour cell(potential donor base station) is much better than the current donorbase station.

In general, and unless there is a clear intention to the contrary,features described with respect to one aspect of the invention may beapplied equally and in any combination to any other aspect, even if sucha combination is not explicitly mentioned or described herein.

As is evident from the foregoing, the present invention involves signaltransmissions between a network and terminals in a wirelesscommunication system. In a wireless communication system, typically,wireless access to the network is provided by one or more base stationsor access points. Such a base station may take any form suitable fortransmitting and receiving such signals. It is envisaged that the basestations will typically take the form proposed for implementation in the3GPP LTE and 3GPP LTE-A groups of standards, and may therefore bedescribed as an eNB (eNodeB) (which term also embraces Home eNB or HeNB)as appropriate in different situations. However, subject to thefunctional requirements of the invention, some or all base stations maytake any other form suitable for transmitting and receiving signals fromuser equipments.

Similarly, in the present invention, each subscriber station may takeany form suitable for transmitting and receiving signals from basestations. For example, the subscriber station may be referred to as auser equipment (UE) or mobile station (MS), and may take any suitablefixed-position or movable form. For the purpose of visualising theinvention, it may be convenient to imagine the terminal as a mobilehandset (and in many instances at least some of the user equipments willcomprise mobile handsets), however no limitation whatsoever is to beimplied from this. In the detailed description which follows, in whichembodiments of the present invention are described with respect to LTEby way of example, the terminal is referred to as a UE in accordancewith usual LTE terminology.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is made, by way of example only, to the accompanying drawingsin which:

FIG. 1 shows a conventional (non-relay) system architecture in LTE;

FIG. 2 shows a system architecture including mobile relays in LTE;

FIG. 3 shows the protocol layers for (1) the User plane and (2) theControl Plane in an LTE system;

FIG. 4 shows bearers in an LTE system;

FIG. 5 shows a conventional handover procedure in LTE;

FIG. 6 shows a handover scenario involving mobile relays;

FIG. 7 shows the principle of a “data plane only” handover in accordancewith the present invention;

FIG. 8 shows a handover procedure in accordance with a first embodiment(“Case 1”) of the present invention;

FIG. 9 shows a handover procedure in accordance with a second embodiment(“Case 2”) of the present invention; and

FIG. 10 shows a handover procedure in accordance with a third embodiment(“Case 3”) of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention may provide a wirelesscommunication method to provide mobility support of UEs that are servedby mobile relay base stations. FIG. 6 illustrates an example of theaddressed problem. Suppose that mobile relay base station 14 MR_1 iscurrently connected to Cell_A provided by a donor eNB 13, and is movingat relatively low speed towards Cell_B provided by another eNB 11.Suppose also that a UE 12 is attached to MR_1 and in connected mode, buthas a low speed relative to MR_1. While MR_1 is moving away from the UE12, handing over to another cell is required for this UE 12 in order tokeep its connectivity with a certain QoS guarantee.

If we assume further that the traffic load is very high in Cell_A, itwould be beneficial if this UE 12 can be handed over to another mobilerelay station MR_2 or MR_3 (that is acting as a capacity booster). Thispresents a handover situation which is more complex than that shown inFIG. 3. For example, there may be no X2 interface available forcommunication between the two mobile relay base stations, even if theyare served by the same donor eNB; consequently handover may have toproceed via the S1 interface. A further complication is that both MRs(source and target) will typically be moving. Due to complexity of thissituation, it may require a long latency time to hand over the UEbetween two mobile relay base stations (which may be attached todifferent donor base stations), and this may not be not acceptable interms of QoS requirements. An efficient handover scheme is required inorder to solve this issue.

Embodiments of the present invention can allow handover latencyreduction in the case that a UE that is attached to a mobile relay andis losing the signal. Rather than connecting to the donor it connects toanother mobile relay (assuming sufficient signal strength). Theprinciple of the invention will now be explained, followed by specificembodiments reflecting various handover situations.

A feature of the present invention is that it enables handover of datatraffic to be performed separately from control signalling, which isdealt with separately. The control plane may be unaffected by such ahandover. This is referred to below as a “data plane handover” for a UEor a group of UEs from a mobile relay base station to a target basestation.

Referring to FIG. 7, a UE 12 connects to a mobile relay base station(MR_1′) that is connected to a donor base station 13 (Cell_A) forbackhaul connectivity (as shown in FIG. 4). The control plane signallingis handled by the donor base station 13 (so for example the S1 interfaceis terminated between donor eNB and MME), while the mobile relay basestation MR_1′ serves the UE 12 for its data traffic delivery at bothuplink and downlink (in a manner somewhat analogous to CoMP or CA). Inother words, the UE 12 has a “direct” link with the donor eNB 13 forcontrol signalling, in addition to a relay link with MR_1′ for datatraffic.

Like a conventional handover, a data plane handover needs to beperformed when the quality of radio connection between the UE 12 and themobile base station MR_1′ becomes deteriorated, which may be due to themobile relay moving away from the UE or vice versa. It should be notedthat whilst the principle of separation of control plane and user planeis well-known and indeed inherent to the LTE standard as indicated inFIG. 3, such a data plane handover (i.e. without also transferring thepath for control signalling) is not conventional.

It may be assumed that each cell (actual or potential donor eNBs 13 and11) is broadcasting reference signals, which the UE or mobile relay basestation may use to assess the signal quality with respect to each cell.Considering the scenario shown in FIG. 7, in which it is desired tohandover the UE 12 from MR_1′ to MR_2, the overall handover schemeconsists of the following steps:

1. The UE performs measurements of its serving cells (e.g. MR_1′ andCell_A in FIG. 7) and neighbouring cells (e.g. MR_2 in FIG. 7), andreports the measurement results to the network when, for example, thesignal quality at the UE of a neighbour cell (Cell_B) is better than thesignal quality at the UE from the current serving cell. With respect toCell_B, the measurement procedure may be similar to that adopted inCoMP/CA (Coordinated Multipoint/Carrier Aggregation) for example.

Meanwhile, the current serving cell MR_1′ monitors the link quality tothe donor base station (Cell_A) and signals (such as a cell-specificreference signal, CRS) from the neighbouring cells (e.g. Cell_B in FIG.4), as well as the link quality to the UE by measuring, for example,sounding channel of the UE. Handover may be triggered when certaincondition is met, such as

-   -   1) Based on UE's measurement report, the signal quality of a        neighbour cell is much better than the current serving cell        (i.e. MR_1′). (FIG. 8 illustrates the overall message flow of        this case)    -   2) Based on the measurement of the serving mobile relay base        station, the signal quality of the link to the UE is worse than        a certain threshold. (FIG. 9 illustrates the overall message        flow of this case)    -   3) Based on the measurement of the serving mobile relay base        station, the signal quality of a neighbour cell (i.e. Cell_B) is        much better than the current donor base station (i.e. Cell_A).        (FIG. 10 illustrates the overall message flow of this case).

The subsequent procedure depends upon which of the above Cases 1)-3)applies. Thus, each of the following Cases constitutes a differentembodiment of the present invention.

Case 1)

2.1. In case 1), the UE sends measurement reports to the serving eNB(e.g. Cell_A in FIG. 8) that handles control signalling.

3.1 Cell_A makes HO decision to initiate the handover procedure towardsthe most suitable neighbour cell MR_2. Because the Cell_A continueshandling this UE's control signalling, only data traffic needs to behanded over to the target cell. For this purpose, Cell_A sends cellactivation request to MR_2 (that is a mobile relay currently connectedthe Cell_A), in which the concerned UE's context is included as well asthe current serving cell for data traffic MR_1′.

4.1 MR_2 performs admission control, and sends acknowledgement toconfirm the handover.

5.1 Cell_A sends de-activation request to MR_1′ asking MR_1′ to handoverthe data traffic of the concerned UE to the target cell MR_2. MR_1′acknowledges the request.

6.1 Cell_A sends data plane only handover command to the UE and asks theUE to connect to target cell MR_2 for data traffic transmission.

7.1 Meanwhile, MR_1′ prepares the data traffic handover of the concernedUE by sending the SN status report to the target cell MR_2 indicatinguplink PDCP SN receiver status and the downlink PDCP SN transmitterstatus associated with the concerned UE's data traffic. In addition, GTPtunnel will be set up between the source and target cells for datatraffic forwarding.

8.1 Upon receipt of the data plane only HO command, the UE detaches fromthe source cell MR_1′ and connects to target cell MR_2 for data traffictransmission. The UE sends out data plane path switch complete messageto the serving base station to indicate the completion of this handoverprocedure. Meanwhile, the UE maintains its “direct” link with the donoreNB (Cell_A) for control plane signalling.

Case 2)

2.2. In case of 2), the MR_1′ sends measurement reports to the servingeNB (e.g. Cell_A in FIG. 9) that handles control signalling when thesignal quality of the link to the UE is worse than a certain threshold.

3.2 Cell_A makes HO decision based on the measurement report for MR_1′concerning the UE. Taking into account of the UE's report, Cell_Ainitiates the handover procedure towards the most suitable neighbourcell MR_2. Because the Cell_A continues handling this UE's controlsignalling, only data traffic needs to be handed over to the targetcell. For this purpose, Cell_A sends cell activation request to MR_2(that is a mobile relay currently connected the Cell_A), in which theconcerned UE's context is included as well as the current serving cellfor data traffic MR_1′.

4.2-8.2. These are the same as steps 4.1-8.1 mentioned above.

Case 3)

FIG. 10 shows the process of handover by Cell_A of UEs currently servedboth by itself (for control plane signalling) and by MR_1′ (for datatraffic transmission). Such handovers need to take place before Cell_Ahands over MR_1′ itself to Cell_B (not shown here).

2.3. In case of 3), the MR_1′ sends measurement reports to its donorbase station when the signal quality of a neighbour cell (i.e. Cell_B inFIG. 10) is much better than the current donor base station (i.e.Cell_A).

3.3 Before Cell_A starts handover preparation for the MR_1′ towardsCell_B, it needs to handover to MR_2 the UEs currently served by bothitself (for control plane signalling handling) and MR_1′ (for datatraffic transmission). Cell_A makes HO decision for such UEs based onthe measurement report from the UE and initiates the handover proceduretowards the most suitable neighbour cell MR_2. Because the Cell_Acontinues handling this UE's control signalling, only data traffic needsto be handed over to the target cell. For this purpose, Cell_A sendscell activation request to MR_2 (that is a mobile relay currentlyconnected the Cell_A), in which the concerned UE's context is includedas well as the current serving cell for data traffic MR_1′.

4.3-8.3. These are the same as steps 4.1-8.1 mentioned above.

Note: in this case, Cell_A may initiate the handover preparationprocedure for the MR_1′ towards target cell Cell_B in parallel. Suchhandover may take place in the conventional manner, and consequently isnot shown here.

Thus, to summarise, an embodiment of the present invention relates to ascheme intended to 1) provide mobility support of user equipments thatare served by mobile relay base stations; 2) provide handover of userequipment for data traffic only between two base stations. This isachieved by information exchanging between the source and target mobilerelay stations of the concerned UE in handover, and/or the correspondingdonor base stations if applicable.

The handover is triggered by a combination of one or more of: 1) basedon UE's measurement report, the signal quality of a neighbour cell ismuch better than the current serving cell; or 2) based on themeasurement of the serving mobile relay base station, the signal qualityof the link to the UE is worse than a certain threshold; or 3) based onthe measurement of the serving mobile relay base station, the signalquality of a neighbour cell is much better than the current donor basestation.

Various modifications are possible within the scope of the presentinvention.

Reference has been made above to mobile relay base stations. However, itis not essential for the relay stations to be mobile. Embodiments of thepresent invention may also be advantageously applied with fixed relaystations, or with a mixture of fixed and moving relay stations.

Whilst it is not essential for each relay station to have full basestation functionality as implied by the term “mobile relay basestation”, it is necessary for each relay station to which the presentinvention is applied to have both user plane and control planefunctionality, so that the user data traffic can be handled differentlyfrom the control plane signalling.

The above embodiments have described a “lossless” handover by way ofexample. However, the present invention is equally applied to aso-called “seamless” handover, in which priority is given to the speedof handover (minimum latency) rather than to avoiding loss of packets.In such a case, the procedure will be the same as described with respectto FIGS. 8 to 10 except that there is no need for the SN Status Reportand GTP Tunnel.

Reference is made above to a “direct” link between the UE and donor eNBfor the control signalling, by which is meant a wireless link which doesnot include any mobile relay base station. It is not essential for thisto be a single hop from UE to donor eNB. A fixed relay station, forexample, if it provides a sufficiently robust connection, could beinterposed in this “direct” link. Consequently the claims refer to a“first link” between the subscriber station (UE) and base station (donoreNB), the term “second link” being used to refer to wireless links viaone or more (mobile) relay stations (MRs).

Any of the embodiments mentioned above may be combined in the samesystem. The same eNB may operate in accordance with more than one of theembodiments simultaneously, and one UE may likewise operate inaccordance with more than one of the embodiments simultaneously.

Whilst the above description has been made with respect to LTE andLTE-A, the present invention may have application to other kinds ofwireless communication system also. Accordingly, references in theclaims to “user equipment” are intended to cover any kind of subscriberstation, MTC device and the like and are not restricted to the UE ofLTE.

In any of the aspects or embodiments of the invention described above,the various features may be implemented in hardware, or as softwaremodules running on one or more processors. Features of one aspect may beapplied to any of the other aspects.

The invention also provides a computer program or a computer programproduct for carrying out any of the methods described herein, and acomputer readable medium having stored thereon a program for carryingout any of the methods described herein.

A computer program embodying the invention may be stored on acomputer-readable medium, or it may, for example, be in the form of asignal such as a downloadable data signal provided from an Internetwebsite, or it may be in any other form.

INDUSTRIAL APPLICABILITY

The invention allows efficient handover in mobile relay base stationscenarios in a 3GPP network, e.g. UMTS or LTE network. This is achievedby enabling the user plane data traffic only handover between two basestations. The fields of application of this invention include all wiredand wireless communications systems where relaying techniques are used.

What is claimed is:
 1. A handover method in a wireless communicationsystem having a subscriber station (12), at least one base station (13)and a plurality of relay stations (14, MR_1′, MR_2), the subscriberstation exchanging data traffic and signalling by employing wirelesslinks with the base station (13), said wireless links including relaylinks which are indirect links between the subscriber station (12) andthe base station (13) via one or more of the relay stations (14, MR_1′,MR_2), the method comprising: measuring a signal quality of each of aplurality of said wireless links; determining a need for a handover ofthe subscriber station (12) from a serving relay station (MR_1′) toanother relay station (MR_2) by comparing the signal quality of a saidrelay link with a predetermined threshold and/or with the signal qualityof another said wireless link; and performing a handover of thesubscriber station (12) from said serving relay station (MR 1′) to saidanother relay station (MR_2); characterised by: employing, for controlsignalling between the subscriber station (12) and the base station(13), a said wireless link which is a direct link between the subscriberstation and the base station; and performing said handover for datatraffic only whilst maintaining said direct link for control signalling.2. The method according to claim 1 wherein at least the serving relaystation (MR 1′) is mobile and the base station (13) acts as a donor basestation for the relay stations.
 3. The method according to claim 1wherein the measuring is performed at the subscriber station (12) andthe determining is performed at the base station (13) in response toreports on said signal quality received from the subscriber station. 4.The method according to claim 1 wherein the measuring is performed atthe serving relay station (MR_1′) and the determining is performed atthe base station (13) in response to reports on said signal qualityreceived from the serving relay station.
 5. The method according toclaim 3 wherein the determining determines whether the signal quality ofa wireless link of the subscriber station (12) with another relaystation (MR_2) or another base station (11) exceeds by a predeterminedthreshold the measured signal quality of a wireless link between thesubscriber station and the serving relay station (MR_1′).
 6. The methodaccording to claim 4 wherein the determining determines whether themeasured signal quality of a wireless link of the serving relay station(MR₁₃ 1′) with the subscriber station (12) is worse than a predeterminedthreshold.
 7. The method according to claim 4 wherein the determiningdetermines whether the signal quality of a wireless link of the servingrelay station (MR_1′) with another base station (11) exceeds by apredetermined threshold the signal quality of a wireless link betweenthe serving relay station and the donor base station (13).
 8. The methodaccording to claim 7 further comprising the relay station which was theserving relay station (MR_1′) itself performing a handover from saidbase station (13) to said another base station (11).
 9. The methodaccording to claim 1 wherein the performing a handover comprises thebase station (13) identifying said another relay station as a targetrelay station (MR_2), sending a cell activation request to the targetrelay station including a context for the subscriber station (12) andidentifying the relay station which currently acts as a serving relaystation (MR_1′) for data traffic of the subscriber station.
 10. Themethod according to claim 9 wherein the performing a handover furthercomprises: the target relay station (MR_2) performing admission controlof the subscriber station (12); the base station (13) sending adeactivation request to the serving relay station (MR_1′); the basestation (13) sending a data traffic only handover command to thesubscriber station (12) and a request to the subscriber station toconnect to the target relay station (MR_2) for data traffictransmission; the serving relay station (MR_1′) sending a status reportto the target relay station (MR_2); the serving relay station (MR_1′)forwarding data traffic to the target relay station (MR_2); and thesubscriber station (12) detaching from the serving relay station (MR_1′)and connecting to the target relay station (MR_2) for data traffictransmission.
 11. A wireless communication system having a subscriberstation (12), at least one base station (13) and a plurality of relaystations (14, MR_1′, MR_2), the subscriber station (12) arranged forexchanging data traffic and signalling by employing wireless links withthe base station (13), said wireless links including relay links whichare indirect links between the subscriber station (12) and the basestation (13) via one or more of the relay stations (14, MR_1′, MR_2),the system comprising: measurement means for measuring a signal qualityof each of a plurality of said wireless links; handover determinationmeans for determining a need for a handover of the subscriber station(12) from one relay station (MR_1′) to another relay station (MR_2) bycomparing the signal quality of a said wireless link with apredetermined threshold and/or with the signal quality of another saidwireless link; and handover performing means for performing a handoverof the subscriber station (12) from said one relay station (MR_1′) tosaid another relay station (MR_2); characterised in that: the subscriberstation (12) is arranged to employ a direct link between the subscriberstation and the base station (13) for control signalling between thesubscriber station and the base station; and said handover performingmeans is arranged for performing a handover for data traffic only whilstmaintaining said direct link for control signalling.
 12. A subscriberstation (12) for use in the system according to claim
 11. 13. A relaystation (14) for use in the system according to claim
 11. 14. A basestation (13) for use in the system according to claim 11.